[Concurrency] Add a debug variable that holds the current version of the concurrency library.

This allows lldb to know which version of the internals layout it needs to use.

rdar://135886268

Author: mikeash

include/swift/ABI/Actor.h

@@ -20,6 +20,12 @@
 #include "swift/ABI/HeapObject.h"
 #include "swift/ABI/MetadataValues.h"
 
+// lldb knows about some of these internals. If you change things that lldb
+// knows about (or might know about in the future, as a future lldb might be
+// inspecting a process running an older Swift runtime), increment
+// _swift_concurrency_debug_internal_layout_version and add a comment describing
+// the new version.
+
 namespace swift {
 
 /// The default actor implementation.  This is the layout of both

include/swift/ABI/Task.h

@@ -43,6 +43,12 @@ class TaskOptionRecord;
 class TaskGroup;
 class ContinuationAsyncContext;
 
+// lldb knows about some of these internals. If you change things that lldb
+// knows about (or might know about in the future, as a future lldb might be
+// inspecting a process running an older Swift runtime), increment
+// _swift_concurrency_debug_internal_layout_version and add a comment describing
+// the new version.
+
 extern FullMetadata<DispatchClassMetadata> jobHeapMetadata;
 
 /// A schedulable job.

stdlib/public/Concurrency/Debug.h

@@ -49,6 +49,12 @@ const void *const _swift_concurrency_debug_task_future_wait_resume_adapter;
 SWIFT_EXPORT_FROM(swift_Concurrency)
 bool _swift_concurrency_debug_supportsPriorityEscalation;
 
+/// The current version of internal data structures that lldb may decode.
+/// The version numbers used so far are:
+/// 1 - The initial version number when this variable was added, in swift 6.1.
+SWIFT_EXPORT_FROM(swift_Concurrency)
+uint32_t _swift_concurrency_debug_internal_layout_version;
+
 } // namespace swift
 
 #endif

stdlib/public/Concurrency/Task.cpp

@@ -87,6 +87,8 @@ const void *const swift::_swift_concurrency_debug_asyncTaskSlabMetadata =
 bool swift::_swift_concurrency_debug_supportsPriorityEscalation =
     SWIFT_CONCURRENCY_ENABLE_PRIORITY_ESCALATION;
 
+uint32_t swift::_swift_concurrency_debug_internal_layout_version = 1;
+
 void FutureFragment::destroy() {
   auto queueHead = waitQueue.load(std::memory_order_acquire);
   switch (queueHead.getStatus()) {